Backscatter x-ray inspection systems have been used for more than 25 years to detect organic materials concealed inside baggage, cargo containers, in vehicles, and on personnel. Because organic materials in bulk preferentially scatter x-rays (by Compton scattering) rather than absorb them, these materials appear as brighter objects in backscatter images. However, because the backscatter intensity is a complicated function of the distance, position, and the shape and thickness of the object, as well as the amount of intervening clutter obscuring the object, it is difficult to use the backscatter intensity to further classify the organic material.
Simulations carried out over many years have attempted to find a reliable way of using dual-energy (or multi-energy) backscatter measurements to assist in identifying concealed materials, other than by just using the measured intensity alone. One method is to alternate the energy of the x-ray source, and to look at the relative backscatter intensity from an object in the backscatter image at the two energies. The ratio R of the low energy intensity to the high energy intensity of backscattered x-rays can be used to classify the object material. Another method is to use a single energy source, but to temporally change the beam filtration so that beams with alternating energy spectra are incident on the object being inspected. Examples are taught, for example, in U.S. Pat. No. 6,249,567 (to Rothschild, et al.) and U.S. Pat. No. 6,459,764 (to Chalmers, et al.), both of which are incorporated herein by reference.
A further method known in the art for illuminating an inspected object with multiple energies is to use a single energy source, and energy-resolving detectors, whereby the energy of each backscatter x-ray photon is characterized when it is detected. In this case, the ratio R may be defined as the detected intensity in an energy range with a lower average energy divided by the intensity in an energy range with a higher average energy.
In each of the foregoing scenarios, it can be shown that the ratio R is a strong function of the distance from the detectors to the illuminated object and a somewhat weaker function of the shape of the illuminated object. This is because the energy of backscattered x-rays depends strongly on the scatter angle, whose range in this case is defined by the solid angle of the detectors relative to the point of scatter. Since the solid angle is a function of the distance D between the detectors and the scatter point, the ratio R is also a function of the distance D. Moreover, since the detectors are typically offset from the beam plane by some distance, the amount of overlying material within the object that the scattered radiation must pass through (from the scatter point to the detectors) also depends on the distance to the object. This effect is very noticeable in a flat sheet of steel, for example, where for an incident 225 kV Bremstrahlung beam, the mean energy of the scattered x-rays incident on the detectors (offset from the beam axis by 12 inches) can vary from 72 keV for scatter from an object a few feet away to only 63 keV for an object at a distance of 10 feet. This means the scattered radiation from a sheet of steel close to the detectors is significantly hardened compared with scattered radiation from a more distant steel sheet.
The dependence of the ratio R on the shape of the illuminated object is due to the varying path length inside the object that the scattered x-rays must traverse before exiting the object and reaching the detectors. For example, this distance is smaller for spherical objects compared with rectangular ones. For objects consisting of higher-Z material (which preferentially absorbs the lower energy x-rays), this will result in a lower value of R for the rectangular objects compared with the spherical ones.
Consequently, a reliable mechanism for using energy resolution of x-ray backscatter properties of an object to classify the object by atomic number has proven elusive.
The general concept of using energy analysis of backscattered x-rays to screen personnel, where the energy spectrum of the x-ray source beam is varied over time by a periodically varying filter, has been taught in U.S. Pat. No. 7,561,666 (to Annis, incorporated herein by reference), however the teachings of Annis are restricted to the case where the scattering object is not screened by an intervening layer of highly attenuating material, while screening by a high-Z material is generally the case where materials are screened in a metal suitcase or a vehicle. At the typical x-ray energies at which x-ray backscatter body scanners operate, 40-110 keV, the classification of materials is necessarily quite crude, with all objects being classified into one of only three or four ranges of atomic number. These might include organic, aluminum, metallic (e.g. steel or copper, etc.), and high-Z (e.g. lead) materials. Typically, moreover, the relative brightness of the backscatter signal is already capable of providing adequate separation between these categories of materials for body scanners, and the energy resolution of the backscatter signal is superfluous.
What would be more useful would be to be able to discriminate between different organic materials—for example, between human tissue and explosives. However, if this is to be accomplished using the techniques taught by Annis, the end-point energy of the lower energy must be very low (only about 30 keV or less), making the practical application of this technique problematic for a practical body scanner, because of the high attenuation of x-rays in this energy range by intervening clothing between the x-ray source and the inspected person's body.
Moreover, the teachings of Annis nowhere address the question of whether highly scattering objects, such as organic materials, lie in front of, or behind, a highly attenuating structural feature such as a sheet of high-Z material, such as a steel car body, for example.